What I would eagerly recommend is, as Didier wrote in your question about colors, that you start by reading one of those books. With this old machines you need to understand the hardware very well in order to do anything.

For instance clearing the screen is a matter of writing empty contents to the memory area where it is mapped. You need to understand this, where it is located (depends on the graphic mode HIRES or TEXT) and how information is stored (bit 7 is inverse flag, bit 6 is 1 for pixels or 0 for an attribute and bits 0-5 are the pixel values (on or off) or the attribute code) so "empty" means binary 01000000. See? You also need to understand how the oric display works (what are attributes?).

You can call the ROM in this case of course, but that won't be always the solution. Specially if you need a very fast or somewhat personalized routine.

The concept of "clearing the screen" is complex on the Oric, because you want to make sure you prepare the screen so you can write stuff later, which means having a clear idea of where you want to put your color attributes. It's also important to specify if you are using the TEXT mode, the HIRES mode, or some hybrid mode that switches from one to the other.